git_revp is something I personally can't stop pronouncing
"rev pointer". I'm sure others would suffer the same
problem.

Also, rename the git_revp_ sub-api "gitrp_". This is the
first of many such renames, primarily done to prevent
extreme inflation in the "git_" namespace, which we'd like
to reserve for a higher-level API.

While we're at it, we remove the noise-char "c" from a lot
of functions. Since revision walking is all about commits,
the common case should be that we're dealing with commits.
Exceptions can get a more mnemonic description as needed.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>